A tunable external cavity laser is a device that allows a user to select the output wavelength of light from a range of wavelengths available from the device. Each tunable external cavity laser has a range, 1500 to 1620 nm for example, of adjustable wavelength selection. The user has an ability to select any one wavelength within the range to amplify and emit from the system.
By rapidly selecting successive wavelengths of light, wavelength modulation of the light is achieved. This wavelength modulation is very useful for spectroscopy (used in chemistry and biochemistry) and optical communication equipment testing. By using wavelength modulation, low frequency noise is eliminated from a test signal. Accordingly, higher rates of wavelength modulation in a tunable external cavity laser are almost always desirable.
The tunable external cavity laser is centered around a gain medium. The gain medium amplifies light in a given wavelength range for a given device and is well known in the art. A commonly used gain medium for an external cavity laser is a laser diode with an antireflective coating. The antireflective coating reduces residual reflections, which encourages single mode operation. A mode refers to a single wavelength of light. A laser diode emits many different wavelengths of light, which are tightly clustered together, 1500 to 1620 nm for example. Laser diodes are sometimes characterized by the center wavelength, which is the wavelength in the center of the wavelength range that the laser diode emits. Other commonly known gain mediums, such as gas, can also be used, but their large size may not be desirable.
In order to amplify a specific wavelength of light, that specific wavelength should be reflected back into the gain medium. In a system where a single output wavelength is desired, it is necessary to select a single wavelength of light to be directed back to the gain medium. One technique is to use a diffraction grating and a retroreflector. This arrangement is known as a Littman configuration.
FIG. 1 shows a prior art example of a tunable external cavity laser 100. A beam 140 of light with multiple wavelengths leaves the gain medium 110 at point A and strikes the diffraction grating 120 at point B. Each wavelength of light diffracts off the diffraction grating 120 at a different angle. A retroreflector 130 is rotated relative to the diffraction grating 120 to select the wavelength of light that is desired to be amplified. Only the light which diffracts off of the diffraction grating 120 at point B and hits the retroreflector 130 at point C such that its path B–C is perpendicular to the retroreflector 130 is reflected by the retroreflector 130 such that its return path is C-B-A. The returning light gets amplified when it traverses again through the gain medium 110. The wavelength of the light that experiences amplification through multiple reflections is determined by the angle between the retroreflector 130 and the diffraction grating 120. The path A–B–C forms the cavity of the tunable external cavity laser 100.
Because only one wavelength of light is reflected back into the gain medium 110, only that wavelength of light is amplified. The light emitted from the gain medium 110, therefore, has one dominant wavelength with insignificant amounts of all of the other wavelengths created by the gain medium 110.
Some of the light emitted from the gain medium 110 reflects off of the diffraction grating 120 and exits the system toward point D. This light comprises the output of the tunable external cavity laser 100. It should be apparent that changing the angle of the retroreflector 130 relative to the diffraction grating 120 results in the amplification, and, therefore, output of a different wavelength of light.
One important aspect of a tunable external cavity laser is that the cavity length should be a constant multiple of the selected wavelength of light. As the output wavelength is tuned from one wavelength to another, the cavity length needs to be changed such that the number of wavelengths in the length of the external cavity laser 100 remains constant. If not, mode hop can occur. Mode hop is an unintended switch from the desired wavelength to a nearby wavelength. The minimization or elimination of a mode hop is highly desired.
FIG. 2A shows a prior art example of a tunable external cavity laser 100 where wavelength selection and cavity length adjustment is achieved in one physical movement. The retroreflector 130 rotates about a pivot 210. The retroreflector 130 rotates relative to the diffraction grating 120 and the distance B–C (hence the cavity length) changes as the retroreflector 130 rotates about the pivot 210. Proper placement of the pivot 210 is critical. Unfortunately, manufacturing costs may be relatively high due to the tight tolerances. In addition, it may be difficult or impossible to ideally match the cavity length to all possible wavelengths emitted by the gain medium 110. Furthermore, the effects of wear during the life of the system and the effects of thermal fluctuation can cause undesired changes in the cavity length, which can lead to mode hop.
Prior art tunable external cavity lasers have used DC servo motors, stepper motors, and rotary voice coil actuators to rotate the retroreflector relative to the diffraction grating. DC servo motors and stepper motors often require transmissions to achieve the desired amount of movement resolution. Unfortunately these transmissions wear out, require significant power to move, and have limited positional accuracy. In addition, DC servo motors and stepper motors do not have the desired speed to attain high frequency modulation. Rotary voice coil actuators, such as those used in hard disc drives, work very well, but they are quite large and limit miniaturization of the device.
FIG. 2B shows an example of a prior art tunable external cavity laser with a rotary voice coil actuator, the type that may also be used in a hard disk drive. The voice coil actuator arm 220 rotates around pivot 210. The retroreflector 130 is attached to the actuator arm 220. The voice coil 230 is also attached to the actuator arm 220. The voice coil 230 interacts with the magnets 240, 242 to control the position of the actuator arm 220.
There is a need for a tunable external cavity laser with a high frequency modulation, excellent wavelength selection accuracy, mode hop free operation, long life, low-cost, and small form factor.